This invention relates to well logging and, more particularly, to an apparatus and method for determining and/or using textural formation properties to obtain improved well logs.
In the well logging art, there are a number of properties of subsurface formations which are considered commercially important, and which have been the object of continuing efforts to identify techniques for improving measurement and interpretation accuracy. For example, a form of the well known Archie equation relates the conductivity of the formation, .sigma., to the conductivity of the water in the formation, .sigma..sub.w, as follows: EQU .sigma.=.sigma..sub.w .phi..sub.w.sup.m ( 1)
where .phi..sub.w is the water-filled porosity of the formation, and m is the so-called "cementation" exponent of the formation, generally considered to be a number around 2. The water saturation of the formation, i.e., the fraction of pore spaces containing water, is expressed as: ##EQU1## where .phi..sub.t is the total porosity of the formation. The hydrocarbon saturation, S.sub.h is the unity complement of the water saturation, i.e.: EQU S.sub.h =1-S.sub.w ( 3)
In many instances, the total porosity, .phi..sub.t, can be determined with relatively good accuracy, so an accurate determination of water-filled porosity, .phi..sub.w, would be particularly valuable in determining water saturation and hydrocarbon saturation. In a situation wherein the formation conductivity has been measured, and, for example, the water conductivity in the measured zone is known (such as in the formation invaded zone or from other inputs), one could use the Archie equation to determine .phi..sub.w, if m were known. However, in many applications the cementation exponent m, which relates to factors that include the structural make-up of the rock, the nature of interconnection of the pores, etc., involves a degree of guesswork in its determination.
From this simplified description, it will be understood that it desirable to devise techniques for more accurately determining the water-filled porosity and the cementation of formations surrounding a borehole, and it is among the objectives of the present invention to set forth such techniques.
It has long been recognized in the well logging art that a knowledge of the dielectric permittivity of a formation surrounding a borehole would provide useful information concering the nature of fluids in the formation, since there is a striking contrast between the dielectric constant of water (about 80) and the dielectric constant of hydrocarbons (about 5). The development of electromagnetic propagation types of devices, such as the electromagnetic propagation tool ("EPT" device) described in U.S. Pat. No. 3,944,910, assigned to the same assignee as the present application, provided an improved technique for determining formation dielectric permittivity, as well as conductivity, at shallow depths of investigation. In applying the measurements obtained with an electromagnetic propagation type of device, it would again be desirable to have more accurate indications of the formation water-filled porosity and cementation which, in the case of the invaded zone, would provide useful information concerning the degree to which available hydrocarbons are capable of being flushed from the rock. Also, it would be desirable to have a means for better determining and interpreting the dielectric permittivity and conductivity as obtained from the electromagnetic propagation type of device, and it is among the further objects of the present invention to provide these improvements.
It has been recognized that the textural nature of the formation matrix can play an important role in affecting the electrical properties of rocks, although there has been an absence of practical application in the well logging art. It is among the further objects of the present invention to devise well logging techniques which can be used for determination of the textural properties of a formation matrix, and to use these properties in obtaining other formation parameters.